In the art of covering roofs with elastomeric membranes, such as EPDM terpolymer membranes, it is common to apply a primer to the membrane or roofing structure before application of an adhesive. Adhesives typically include adhesive tapes or solvent based adhesives. Primers can include those that are pure solvent, i.e., those that do not contain any solids, and those that are blends of asphalt and solvent, or polymer and solvent. Those skilled in the art will recognize that solvent based adhesives and primers are similar in composition and serve similar purposes, except that primers typically contain a lower solids content.
For example, U.S. Pat. No. 4,640,730 teaches a process for adhering roofing membranes using an adhesive composition containing a thermoplastic block copolymer, an aromatic hydrocarbon resin, and a solvent. Once the composition sets, heat must be applied to join the surfaces. It has been found, however, that equal and even application is difficult to achieve during field installations, thereby resulting in an improper and imperfect field seam or lap splice.
U.S. Pat. No. 5,008,311 teaches an elastomeric asphalt compound comprising an asphalt (bitumen) component, an isocyanate prepolymer component, and a volatile compatibilizer component. The isocyanate prepolymer component polymerizes within the asphalt subsequent to application, thereby providing a polymer network within the asphalt that provides strength and cohesion. The use of this adhesive as a seam adhesive is problematic, however, because it fails to develop sufficient or adequate strength and resistance to creep rupture. It is believed that the presence of asphalt causes swelling in the EPDM membrane. This adhesive is also undesirable because it requires moisture to initiate the curing process. Finally, the shelf life of this adhesive is unacceptable, especially after the composition has been exposed to the air.
U.S. Pat. No. 5,234,987 teaches a solvent-base adhesive composition for adhering together EPDM roofing membranes that includes a halogenated copolymer of isobutylene and p-methyl styrene, a thermoplastic rubber comprising a styrene-ethylene-butylene-styrene block copolymer, an end-block resin for the thermoplastic rubber, a tackifier, an accelerator/cure package, and a solvent. This composition exhibits a peel strength of at least 3.0 pounds per lineal inch at room temperature, and at least 3.0 pounds per lineal inch at 212.degree. F. This composition has been found less than desirable for primer solutions, however, because the adhesive lacks surface tack that is essential to seam construction. Also, this adhesive lacks sufficient early green strength, which is especially problematic in high stress details common in roofing application, e.g., corner patch flashings. These details have been known to bridge or come loose after only a few hours.
U.S. Pat. No. 4,603,164 is directed toward a solvent based contact adhesive that includes a halogenated butyl rubber, a pre-crosslinked butyl rubber, a ethylene-propylene-noncongugated diene terpolymer, a thermoplastic petroleum based hydrocarbon feed stock derived from aliphatic monomer resin, and an aliphatic isocyanate. This composition can serve to join cured EPDM membranes and maintain satisfactory creep, cold flow and heat strength characteristics. The organic isocyanate ingredient of this invention functions to provide a cure of the halogenated butyl rubber when exposed to moisture.
Because of the early creep rupture at room temperature, early green strength, and limited shelf life associated with primer adhesive compositions known in the art, there is a need for an improved primer composition.